1. Field of the Invention
The present invention relates to an energy recovery system for offgas exhausted from a fuel cell in a solid polymer type fuel cell system.
2. Background Art
As one type of fuel cell system, there is one in which an alcohol type fuel, such as methanol and methane, and a hydrocarbon type fuel are reformed into a hydrogen-rich fuel gas by a reforming reactor, and this fuel gas and an oxidant gas (for example, air) are supplied to a fuel cell to generate power (Japanese Unexamined Patent Application, First Publication Nos. Hei 5-290865, Hei 7-192742 and Hei 7-240223).
Moreover, as a fuel cell system using a solid polymer type fuel cell, there is one in which a raw fuel of an alcohol type fuel and a hydrocarbon type fuel, such as methanol and gasoline, is heated by an evaporator to make fuel vapor, this fuel vapor is reformed into a hydrogen-rich fuel gas by a reforming reactor, and this fuel gas and an oxidant gas (for example air) are supplied as a reactant gas to an anode electrode side or a cathode electrode side of a fuel cell to generate power, the anode offgas and the cathode offgas exhausted from the fuel cell are guided to a catalytic combustion chamber to burn hydrogen remaining in the anode offgas, and the heat of the generated combustion gas is used as a heat source for the evaporator to evaporate the raw fuel.
With the solid polymer type fuel cell, adequate humidity is required at the time of generating power, and therefore in this fuel cell system, the reactant gas is humidified and supplied to the fuel cell. In addition, in the fuel cell, at the time of power generation by means of an electrochemical reaction of hydrogen and oxygen, water is generated, and this water is generated mainly on the cathode electrode side. Therefore, the cathode offgas is exhausted from the fuel cell with high humidity.
In this fuel cell system, heretofore, the anode offgas and the cathode offgas exhausted from the fuel cell are guided directly to the catalytic combustion chamber and burnt, and the generated combustion gas is directly exhausted to the atmosphere, after subjecting to heat-exchange with the raw fuel in the evaporator.
However, the operating temperature of the solid polymer type fuel cell is about 80xc2x0C., and this temperature is approximately the dew-point temperature of the cathode offgas. Therefore, if the offgas exhausted from the fuel cell is directly introduced to the catalytic combustion chamber, as described above, the offgas is cooled due to heat radiation along the offgas piping for guiding the offgas from the fuel cell to the catalytic combustion chamber. As a result, the moisture in the offgas is condensed to form water, and this water may be introduced to the catalytic combustion chamber together with the offgas. In this case, a part of the quantity of heat generated in the catalytic combustion chamber is consumed as latent heat of vaporization of the water. As a result, there is caused a problem in that the quantity of heat at an adequate temperature level required for evaporating the raw fuel cannot be supplied to the evaporator.
The present applicant has also developed the following technique and filed a patent application (not yet published), in order to improve the starting warm-up in a solid polymer type fuel cell system. This is for accelerating warm-up of the evaporator, and the construction is such that in parallel with the offgas pipe for supplying the offgas to the catalytic combustion chamber, a starting catalyst warm-up apparatus having an electric heater catalyst (hereinafter, abbreviated as xe2x80x9cEHCxe2x80x9d) and a first fuel injection nozzle, and a second fuel injection nozzle which directly injects the fuel to the catalytic combustion chamber, are connected to an upstream portion of the catalytic combustion chamber.
At the time of startup of the fuel cell system, the electric heater of the EHC in the starting catalyst warm-up apparatus is energized and heated, and raw fuel (for example, methanol) is injected from the first fuel injection nozzle to thereby vaporize and bum the raw fuel by the EHC, and the combustion gas is supplied to the catalytic combustion chamber to heat the catalytic combustion chamber. As a result, when the catalyst in the catalytic combustion chamber is heated to a temperature higher than a low activation temperature, the raw fuel is directly injected to the surface of the catalyst in the catalytic combustion chamber from the second fuel injection nozzle, and the air for combustion is supplied to the catalytic combustion chamber via the offgas pipe, to completely burn the raw fuel injected from the second fuel injection nozzle in the catalytic combustion chamber, and the combustion gas is supplied to the evaporator to warm up the evaporator. As a result, early warm-up of the evaporator becomes possible, thereby enabling the fuel vapor to be supplied to the reforming device at an early stage, and early warm-up of the fuel cell system can be further promoted.
However, in the case where the injected amount of the raw fuel from the second fuel injection nozzle is increased for promoting warm-up in this fuel cell system, the raw fuel such as methanol requires a period for vaporization and temperature rise before combustion. Therefore, the time until reaching the burnt condition is long, and there may be a case where the fuel is discharged unburnt from the catalytic combustion chamber. Moreover, since in this case vaporization of the raw fuel is not sufficient, the raw fuel may be accumulated in the directly upstream portion of the catalytic combustion chamber, and flow into the catalytic combustion chamber to cause a hot spot (thermal nonuniformity). From such a reason, it has been difficult to promote warm-up by increasing the injection amount of the raw fuel from the second fuel injection nozzle.
A fuel cell system of the present invention comprises: a solid polymer type fuel cell, to which a reactant gas is supplied to generate power; a combustion chamber which burns offgas exhausted from the fuel cell to generate a combustion gas; a first heating device which heats an object to be heated, by using heat of the combustion gas; and a second heating device which provides heat to the offgas by absorbing heat from combustion gas exhausted from the first heating device, on an upstream side of the combustion chamber.
By having such a construction, even if moisture in the offgas exhausted from the fuel cell is cooled and condensed to form condensed water as a result of heat radiation during the process until reaching the combustion chamber, this condensed water can be heated by the waste heat of the burnt offgas in the second heating device to thereby effect vaporization. Therefore, the condensed water does not flow directly into the combustion chamber. As a result, the situation can be prevented where a part of the quantity of generated heat in the combustion chamber is consumed by the latent heat of vaporization of water. Therefore thereby a necessary quantity of heat at an appropriate temperature level can be supplied to the first heating device.
Moreover, the waste heat in the burnt offgas can be recovered to the offgas, and the recovered quantity of heat can be added to the heat output by the combustion chamber and supplied to the first heating device, thereby enabling promotion of energy saving.
The object to be heated may be raw fuel of the reactant gas, and the first heating device may be an evaporator which evaporates the raw fuel.
By having such a construction, it becomes possible to supply the quantity of heat at an appropriate temperature level required for evaporating the raw fuel in a necessary amount required by the fuel cell. It also becomes possible to reduce the consumption of the raw fuel.
The fuel cell system may further comprises: a warm-up air supply device which supplies air for combustion of the combustion chamber from an upstream side of the second heating device, at the time of system startup; and a warm-up fuel supply device which supplies warm-up fuel to the combustion chamber, at the time of system startup.
By having such a construction, the air for combustion supplied from the warm-up air supply device can be heated by the second heating device, and at the time of startup of the system, the fuel for warm-up supplied from the warm-up fuel supply device can be heated to accelerate vaporization and temperature rise, thereby enabling acceleration of combustion of the warm-up fuel. Moreover, the situation can be prevented where the warm-up fuel supplied from the warm-up fuel supply device is supplied in an unburnt state to the first heating device. Therefore wastage of the warm-up fuel can be prevented. In particular, when the first heating device is an evaporator which evaporates the raw fuel of the reactant gas, early warm-up of the evaporator becomes possible, and early warm-up of the whole fuel cell system also becomes possible.
The combustion chamber may be a catalytic combustion chamber, and there may be provided a starting catalyst warm-up device which warms up the catalytic combustion chamber at the time of system startup, and the warm-up fuel supply device comprises a fuel injection nozzle which directly injects fuel to a catalyst in the catalytic combustion chamber
By having such a construction, the starting catalyst warm-up device can activate the catalytic combustion chamber immediately after startup of the fuel cell system. Moreover, since the warm-up fuel is injected in a sprayed state from the fuel injection nozzle, it becomes possible to further accelerate vaporization and temperature rise of the warm-up fuel, thereby enabling further reduction of the warm-up time.
The catalytic combustion chamber may comprise a catalytic chamber which houses a catalyst and a heating chamber adjacently provided on an upstream side of the catalytic chamber, the second heating device is directly connected to the heating chamber, and the starting catalyst warm-up device and a fuel injection nozzle of the warm-up fuel supply device may be installed, facing the heating chamber.
By having such a construction, the second heating device and the catalytic combustion chamber can be arranged in proximity to each other. As a result, at the time of a warm-up operation, vaporization and temperature rise of the warm-up fuel can be accelerated, thereby enabling further reduction of the warm-up time. Moreover, at the time of a generating operation, water in the vaporized offgas can be supplied to the catalytic combustion chamber without being recondensed. Therefore condensed water can be reliably kept from flowing into the catalytic combustion chamber.
The starting catalyst warm-up apparatus may comprise an electric heater catalyst, and a catalyst warm-up fuel injection nozzle which injects fuel to the electric heater catalyst.
By having such a construction, warm-up of the catalytic combustion chamber at the time of startup can be reliably performed, with a simple construction.